1. Field of the Invention
The invention relates to an electric power steering device that generates steering assist torque in accordance with the steering state of the steering wheel or handle, and a control method thereof.
2. Description of the Related Art
An electric power steering device has an assist controller that has a microcomputer in a main portion. Using the assist controller, the device controls the amount of electricity passed to an electric motor in accordance with the steering state of the steering wheel or handle so as to give a desired steering torque to a steering mechanism. For example, a steering torque sensor that detects the steering torque generated in association with the steering wheel operation of a driver and a vehicle speed sensor that detects the vehicle speed are connected to the assist controller. On the basis of the detection signals from the two sensors, the assist controller computes a target current value of the motor that provides an optimal assist torque, and controls the amount of electricity passed to the motor by feeding back a deviation between the target current value and the value of current that actually flows through the motor (actual current value).
In such an electric power steering device, if the steering wheel is quickly turned leftward or rightward to a stroke end (i.e., a stopper portion that mechanically restricts a terminal end of the steerable range), the rotation of the motor suddenly comes to a stopped state, so that the counterelectromotive force of the motor rapidly disappears. Due to the influence thereof, the current through the motor rapidly increases, resulting in overshooting. To overcome this problem, in an electric power steering device of Japanese Patent Application Publication No. JP-A-11-78919, when the overshooting of current is detected, the control gain of the feedback control is enlarged to reduce the overshooting. In an electric power steering device of Japanese Patent Application Publication No. JP-A-8-34359, when the steering torque is greater than or equal to a predetermined value and the steering rotation speed (steering speed) is less than or equal to a predetermined value, the target current to be passed through the motor is corrected so as to decrease the overload current of the motor.
However, in the device of the Japanese Patent Application Publication No. JP-A-11-78919, since the attainment of the stroke end is determined by detecting that the motor current has actually overshot, the determination is delayed, allowing excessively large torque to occur. The device of Japanese Patent Application Publication No. JP-A-8-34359 judges whether the stroke end has been reached on the basis of the steering rotation speed. However, a situation where the steering rotation speed lessens occurs not only in the case where the stroke end is struck, but also in the case where the steering is held. It is necessary to detect the continuation time of a rotation stopped state in order to determine the presence of a steering-held state. Therefore, the timing of preventing the overcurrent is correspondingly delayed. However, if the determination regarding the stroke end is performed earlier, the likelihood of false detections increases. Such a false detection leads to the reduction of the assist torque during the steering-held state, so that good steering assist cannot be obtained.